1. Field of the Invention
The present invention relates to a batch process and an apparatus for producing sugar crystals from syrup in a discontinuously operated crystallization apparatus comprising a heating calandria, which comprises the steps of graining a batch of concentrated syrup by adding seed crystals thereto to form a predetermined volume of magma causing sugar crystals in the syrup to grow by subjecting the magma to evaporation in heat exchange with the calandria and adding syrup while maintaining the mother liquor at a desired level of supersaturation to obtain massecuite, and increasing the brix of the massecuite to a desired value to obtain a final volume of massecuite.
A description of the evaporation of sugar juice to produce syrup and the subsequent boiling of the syrup to produce massecuite has been described in Handbook of Cane Sugar Engineering, by E. Hugot, Elsevier Publishing Company, 1960, particularly pages 348-528.
2. Description of the Prior Art
Apparatus of this type comprises a closed vessel, whose interior is under sub-atmospheric pressure, a heating calandria at the bottom of the vessel, and a mechanical agitator for accelerating the circulation of the syrup fed into the vessel through the calandria. During each production cycle,the closed vessel is filled with a sufficient volume of syrup to cover the calandria at the bottom of the vessel, this volume of syrup is concentrated to bring it to a state of supersaturation and the syrup is grained by introducing a metered quantity of seed crystals to produce seeding magma (mixture of crystals and mother liquor), the sugar crystals in the magma are caused to grow by adding fresh syrup to compensate for the crystallization of the sugar in the mother liquor and the evaporation of water while maintaining the mother liquor at a state of supersaturation until the resultant massecuite attains a predetermined level in the pan, and finally the massecuite is densified to increase its brix value (content of dry substance) to the desired level.
Conventionally, vertical cylindrical vessels equipped with a calandria of low height (less than 1.5 m) have been used for boiling the syrup to reduce the volume of the seeding magma. During the phase of crystal growth, the level of the massecuite in the vessel rises progressively and, at the end of the boiling, the height of the massecuite above
the upper plane of the calandria reaches about 1.5 m to 2 m, depending on whether the massecuite is subjected to natural circulation through the calandria or whether the vessel is equipped with a mechanical agitator to aid in the circulation. In the conventional apparatus, the volume of the seeding magma constitutes about 25% to 35% of the final volume of massecuite.
Because of the considerable height of the massecuite extending above the calandria, the static pressure in the lower part of the vessel is elevated and the boiling temperature of the mother liquor considerably exceeds the average temperature of the massecuite, which has two disadvantages:(1) it is necessary to supply the steam at elevated pressure and temperature to the calandria, and (2) there is a risk of partially redissolving the sugar crystals when they are recirculated to the bottom of the vessel. Furthermore, the agitation in the volume of massecuite above the calandria is weak, even in apparatus equipped with a mechanical agitator, and the massecuite therefore lacks homogeneity, particularly as far as the grain size of the sugar crystals is concerned.